There are many instances when fluid streams, such as the outside-air and exhaust ducts, are not contiguous, so a conventional heat-recovery unit cannot be used. In such cases, where several thousand cubic feet per minute of conditioned air is exhausted, a more elaborate system can be used to recover some of the waste heat. A heat-transfer coil is installed in the exhaust- and outside-air streams, and fluid is circulated between them by a small in-line pump.
The pump is interlocked with the supply fan (which itself is often interlocked with the exhaust-air fan; either way, the controls pump should be configured to operate only when both the exhaust and supply fans are on, to ensure airflow across both coils). A delay in the morning is appropriate, until the exhaust air is true conditioned air.
Otherwise, the system will be doing more harm than good for a while. Other heat-reclaim systems don’t have this flexibility and will actually waste energy on startup, although it helps if the building has a morning warmup cycle (when the outside-air supply is closed off until the building temperature reaches setpoint).
If the return fluid temperature from the outside-air coil is near freezing, condensation on the exhaust can freeze. The bypass valve will prevent this from happening, by tempering the water to some value well above freezing.
This system has applications other than in the recovery of thermal energy from the exhausted airstream. One use is in the recovery of waste heat from a boiler stack, to preheat the outside air delivered to the boiler or to preheat the boiler fuel itself. Waste heat from individual processes can be captured by a heat-reclaim loop, be it embodied in either air or fluid mediums. This thermal energy can be used to boost the efficiency of either the HVAC system or some other industrial process.
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Excerpt from Energy Conservation Projects.